1. Field of the Invention
The present invention relates generally to active-pixel sensors and more particularly to an improved sensor cell having a soft saturation circuit.
2. Description of the Related Art
An important component in an image capture device is an imager integrated circuit that converts incident light into a representative electrical signal. A key component in an imager integrated circuit is a sensor array that includes a plurality of sensor cells arranged in rows and columns. The performance of each sensor cell largely determines the quality of a captured image. Consequently, by improving the sensor cell architecture, the quality of the captured image can be increased.
FIG. 1 illustrates an active pixel sensor (APS) cell 10 employed in conventional digital image capture devices. The active pixel sensor cell 10 includes a photodiode 12 coupled to a RESET transistor 14 and a source follower 16. The RESET transistor 14 selectively resets a pixel node 18 to a predetermined voltage. When a READOUT signal is asserted, the source follower 16 provides the voltage of the pixel node 18 to an image capture system (not shown). Incident light causes the pixel node 18 voltage to drop by collection of electrons photogenerated in the diode space-charge region.
A disadvantage of the cell illustrated in FIG. 1 is that during high light conditions in an array of cells of this type, many of the pixel cells are saturated, resulting in a picture having pixel cells that are washed-out. In other words, all pixels having an amount above a specified amount of incident light appear the same and any detail or contrast between these pixels is lost.
One approach to prevent saturation of the image is to shorten integration time. However, when the integration time is shortened, the sensitivity of the cell also decreases. For example, resolution and detail between pixel cells having low light dramatically decreases. Instead of a washing-out of the picture, the entire scene is dimmed. Moreover, contrast between objects and/or details of low light pixels are lost.
FIG. 2 illustrates a scene presented for capture to an image capture device. This scene includes the sun in the upper left hand corner, an airplane in the upper right hand corner, a building having a shadow in the lower left hand corner and a person standing in the shadow of the building also on the lower left hand corner.
In order to capture the building, the shadow and the person standing in the shadow, the integration time is lengthened. However, the result of lengthening the integration time is that the sun and the airplane are lost as the pixels having greater than a predetermined amount of incident light are saturated, and the picture is washed-out so that only the shadowed parts have contrast in the image.
When the integration time is shortened, the sun and the airplane are resolved. However, the shadow and the person, standing in the shadow, are lost. The shadow and the person do not appear in the image because the length of the integration time is not sufficient to resolve and/or capture the detail of those objects in the scene that are relatively dark as compared to other points in the scene.
Accordingly, there remains a need in the industry for an improved active-pixel sensor cell that can resolve detail of low light pixels and yet at the same time provide details and resolution of high light pixels.